The immune response to HIV infection is poorly understood. Because of the tropism of the virus for cells of the immune system, it is not clear whether certain immunologic alterations reflect a protective host response to HIV or are indicative of immunopathology. Several lines of evidence suggest that there is protective immunity to HIV, particularly in HIV-exposed children. This evidence, along with the enormous social and ethical pressure to blunt the devastating epidemic of HIV in women and children, presents a rationale for ACTG-sponsored immune based therapies for amelioration and prevention of HIV disease. The in vitro testing of cellular and humoral immune function is essential for evaluation of the impact of experimental immune based therapies for HIV infection. The UCLA Pediatric ACTU has the capability to test host immune function through a consortium of immunology laboratories in the Center for the Health Sciences. The laboratories have the capability to: 1) Characterize the level of HIV-specific cellular immunity in response to vaccine or other immunomodulatory therapy utilizing assays of lymphocyte proliferation to HIV proteins and cytotoxicity of T cells to HIV-expressing target cells, 2) Assess HIV-specific humoral immunity in response to active or passive immunization utilizing assays for neutralizing antibody and antibody-dependent cellular cytotoxicity, 3) Assess general cellular immunity by non-HIV specific antigen responses utilizing assays of lymphocyte proliferation to mitogens and non-HIV microbial antigens, 4) Characterize decreased immune activation as a marker of therapeutic efficacy utilizing assays for flow cytometric determination of cell surface activation antigen expression, serum cytokine and soluble activation molecule levels, and cellular expression of mRNA for cytokines. The assays are performed according to available approved or recommended ACTG protocols. The expertise to perform these assays is available at UCLA along with the leadership evidenced by the participation of the Investigators in numerous Immunology and Pediatric Working Groups and Committees within the ACTG.